This invention relates to an amplitude-limiting circuit for preventing signals of amplitudes exceeding a prescribed value from being transmitted to any succeeding circuit.
A conventional circuit of this sort comprises: an operational amplifier having an inverted input terminal, a non-inverted input terminal connected to a ground point and an output terminal; a first resistor having one terminal receiving an input signal and the other terminal connected to the inverted input terminal; a second feedback resistor connected between the inverted input terminal and the output terminal; and a pair of zener diodes connected in series in mutually opposite polarities between the inverted input terminal and the output terminal. (See Jerald G. Graeme et. al., OPERATIONAL AMPLIFIERS; Design and Application, pp. 244-245, McGraw-Hill Book Company, 1971). In such circuit construction, the limited amplitude is usually varied by altering the resistance of the two resistors.
However, such conventional amplitude limiting circuits suffer from the following disadvantage. This construction, in which the input signal is fed to the amplifier through the variable first resistor and further is connected to the output signal through the variable second feedback resistor invites changes in the input-output level diagram of the amplitude-limiting circuit at the normal level. A change in the level diagram of the amplitude limiter necessitates an extremely complex adjustment of the level diagrams of all the relevant components of the transmission system.